1. Field of the Invention
This invention relates to a process for forming the field dielectric regions in integrated circuit structures to thereby prevent inversion in the semiconductor substrate when electrically conducting materials are formed over the surface of said field dielectric regions.
2. Prior Art
Two techniques typically have been used by prior art semiconductor processing technology to fabricate the field dielectric regions in semiconductor structures. Unfortunately, both of these prior art techniques suffer from certain disadvantages. According to one technique a layer of field oxide is grown to the necessary thickness directly on the substrate, and the required field geometry is then achieved by etching the oxide. Unfortunately, it is difficult to etch the oxide with sufficiently precise control, particularly when small geometries are desired.
According to another prior art technique, silicon nitride is used as an oxidation mask for oxidation of the substrate to form thick regions of silicon dioxide. This technique suffers from the disadvantage of creating what are referred to as "bird beaks." The bird beaks are tapering regions of silicon dioxide which occur at the edge between the silicon dioxide field isolation regions and the silicon nitride oxidation mask. This tapering region of silicon dioxide tends to lift the silicon nitride masking layer away from the surface of the underlying substrate, making precise dimensional control difficult. An example of a prior art structure utilizing this technique is shown in FIG. 7. Note the lateral penetration X of the silicon dioxide beneath the silicon nitride layer. This penetration makes accurate control of the dimensions of the device region, designated Y in FIG. 7, difficult to achieve.
One further technique for generating the field dielectric has been attempted, but found generally impractical. According to this technique the field dielectric is created by vapor depositing silicon dioxide regions on the surface of the wafer and then suitably patterning them. Unfortunately, it has been found extremely difficult to fabricate such layers of silicon dioxide on the wafer surface with sufficient purity. Typically the vapor deposited silicon dioxide will have sodium impurities contained therein which defeat the function of the field dielectric. This technique has therefore never been widely used, and generally results in lower yields than the previously mentioned two techniques.